This proposal focuses on the structure and cell surface organization of two endogenous lectins, discoiodins I and II, which are believed to be primary determinants of the morphogenetic cell-cell cohesion system of Dictyostelium discoideum. The results of our previous immunochemical and biochemical studies indicate the existence of regions common to discoidins I and II, as well as the reguions unique to each. This suggests analogies with other families of recognition molcules in which common and unique regions are segregated to form spearate domains responsible for effector and recognition functions respectively. We propose to continue our structural; studies to test whether an analogous situation holds for the discoidins. Specifically, we wish to learn whether the common and unique regions are segregated to form discrete structural domains which are associated with the known functional properties of the discoidins. If true, these findings will have important functional iplication for these cell-cell recognition molecules. Another critical question concerns the organization of the discoidins at the cell surface, particularly with respect to two other putative cohesion molecules, Contact site A and gp150. If these molecules are direct components of the cohesion apparatus, they should directly interacts with one another, either within the plane of the membrane of a given cell, or by binding together the surfaces of adjacent cells We will address these questions through co-capping studies with fluorescent antibodies, immunolactoperoxidase labeling studies, and experiments with chemical crosslinking agents. If successful, these studies will enable us to formulate a more specific model of the cell-cell recognition and cohesion process.